1. Field of the Invention
The present invention relates to a silicon single crystal and to a process for producing it, in which the single crystal is pulled from a melt using the Czochralski method. This method has long been known and is used on an industrial scale to produce semiconductor wafers which in turn form the base material for the fabrication of electronic components.
2. The Prior Art
It is also known that the production of single crystals which have a diameter of 200 mm or greater imposes particular demands. In particular, there are considerable difficulties with deliberately setting the radial crystal properties within the narrowest possible range. This is true in particular for the concentration of foreign substances or dopants and crystal defects or agglomerates thereof. The radial crystal properties are substantially determined by the thermal conditions at the solidification interface and the concentrations of substances which are present there. Heat sources are the heaters used and the heat of crystallization released during solidification. The latter, for example in the case of a silicon single crystal with a diameter of 300 mm at a pulling speed of 0.4 mm/min, is thereby responsible for about 2 kW of additional heat being produced at the solidification interface.
In addition to the direct radiation and heat conduction, the heat transfer processes produced by the melt flows are also of considerable importance. The dissipation of heat in the region of the solidification interface is crucially determined by the heat which is radiated out and the dissipation of heat by conduction in the single crystal. Overall, therefore, the heat balance can be adjusted by means of the structure of the pulling installation, i.e. the geometric arrangement of the heat-conducting parts, of the heat shields and by means of additional heat sources. However, the process conditions, such as for example growth rate, pressure, quantity, type and guidance of purge gases through the pulling installation also make a considerable contribution to the heat balance. By way of example, a temperature reduction is achieved by increasing the pressure or the quantity of purge gas. Higher pulling speeds increase the heat of crystallization which is generated.
Adjusting the heat-transferring melt flows often proves difficult, since complete theoretical calculation in advance is very complicated. The melt flows are dependent on the magnitude and direction of the rotations of the crucible and of the single crystal. For example, rotation in the same direction results in a completely different convection pattern than rotation in opposite directions. Rotation in opposite directions is generally preferred, leading in relative terms to less oxygen-rich material and being more stable over the pulled length of single crystal. The melt flows can also be influenced by the action of forces from electromagnetic fields which are applied. Static magnetic fields are used for slowing-down purposes, while dynamic fields can deliberately change and increase both the magnitude and the direction of the melt flows.
The radial temperature distribution in the solidification region of the single crystal is substantially determined by the heat which is radiated out at the edge. Therefore, the temperature drop is generally very much greater at the edge of the single crystal than in its center. The axial temperature drop is generally denoted by G (axial temperature gradient). Its radial variation G(r) is a very significant factor in determining the crystal internal point defect distribution and therefore also the further crystal properties. The radial change in the temperature gradient G which results from the heat balance is generally determined from digital simulation calculations. For this purpose, to check the calculations, axial longitudinal sections are taken through the single crystal. The radial profile of the solidification interface is made visible by suitable preparation methods. A solidification interface which is bent significantly upward is generally found. A more shallow form indicates a more homogenous temperature gradient. The radial variation of the temperature gradient can be derived more accurately from the behavior of the radial crystal defect distribution for various growth rates.
With regard to the formation of crystal defects, the ratio v/G(r) is of primordial importance, G(r) representing the axial temperature gradient at the solidification interface of the single crystal as a function of the radial position in the single crystal. The variable v represents the speed at which the single crystal is pulled from the melt. If the ratio v/G is above a critical value k1, predominantly vacancy defects occur, which may agglomerate and can then be identified, for example, as COPs (crystal originated particles). Depending on the detection method, they are sometimes also referred to as LPDs (light point defects) or LLS. On account of the generally decreasing radial profile of v/G, the COPs are most prevalent in the center of the single crystal. They generally have diameters of approximately 100 nm and may cause problems during component fabrication. The size and number of the COPs are determined from the starting concentration, the cooling rates and the presence of foreign substances during the agglomeration. The presence of nitrogen, for example, causes the size distribution to be shifted toward smaller COPs with a greater defect density.
If the ratio of v/G is below a critical value k2, which is lower than k1, predominantly silicon internal point defects in the form of interstitials (silicon self-interstitials) appear. These can likewise form agglomerates and manifest themselves on a macroscopic scale as dislocation loops. These are often referred to as A swirl, or the smaller form as B swirl, or as Lpit defects (large etch pits) for short, on account of their appearance. In terms of their size, Lpits are in a range of over 10 μm. In general, even epitaxial layers can no longer cover these defects without any flaws. Consequently, these defects may adversely affect the yield of components.
The range in which neither agglomeration of vacancies nor agglomeration of interstitials takes place, i.e. the range in which v/G is between k1 and k2, is referred to in the broadest sense as the neutral zone or perfect. However, a further distinction is drawn between a range in which unagglomerated vacancies which are still free are located and a region defined by interstitials. The vacancy range, also known as the v region (vacancies), is distinguished by the fact that, given a sufficiently high oxygen content in the single crystal, oxygen-induced stacking faults are formed there, while the i-range (interstitials) remains completely free of flaws. In the narrower sense, therefore, only the i-region is actually a perfect crystal region.
Large oxygen precipitations with a diameter of over 70 nm can be made visible as oxygen-induced stacking faults (OSFs). For this purpose, the semiconductor wafers which have been cut from the single crystal are prepared by means of a special heat treatment referred to as wet oxidation. The size growth of the oxygen precipitates which have been formed during the crystal growth process and are sometimes also referred to as as-grown BMDs (bulk micro defects), is promoted by the vacancies in the silicon lattice. Therefore, OSFs are found only in the v range.
The single crystal becomes virtually defect free if the growth conditions are successfully set in such a way that the radial profile of the defect function v/G(r) lies within the critical limits of the COP or Lpit formation. However, this is not easy to achieve, in particular if single crystals with a relatively large diameter are being pulled, since the value of G is then significantly dependent on the radius. In this case, the temperature gradient at the edge of the crystal is very much higher than in the center, on account of thermal radiation losses.
The radial profile of the defect function v/G(r) or of the temperature gradient G(r) leads to the possibility of a plurality of defect regions being present on one semiconductor wafer cut from the single crystal. COPs preferentially occur in the center. The size distribution of the agglomerated vacancies results from the cooling rate of the single crystal in the region of the solidification interface. The size distribution of the COPs can be deliberately changed from a small number of large COPs to a large number of small, less disruptive COPs by using a high cooling rate or by doping the melt with nitrogen. The COP region is adjoined by the oxygen-induced stacking fault ring (OSF), as a result of the interactions between silicon vacancies and oxygen precipitations. This is followed on the outer side by a completely defect-free region which in turn is delimited by a region with crystal defects comprising silicon interstitial agglomerates (LPITs). At the edge of the single crystal, the interstitials diffuse out as a function of the thermal conditions, so that a defect-free ring in the centimeter range can once again form at that location.
The crystal defect regions which occur in connection with the radial v/G profile are extensively explained in Eidenzon/Puzanov in Inorganic Materials, Vol. 33, No 3, 1997, pp. 219-255. This article also refers to possible ways of producing defect-free material. Reference is made in this context both to the required cooling rates in the agglomeration temperature range, to the influence exerted by means of nitrogen doping and to methods such as the oscillating growth rate. To a certain degree, v/G(r) can be homogenized over the crystal diameter by using passive or active heat shields in the region of the solidification interface, as has been presented, for example, in patent literature EP 866150 B1or U.S. Pat. No. 6,153,008. However, homogenizing the temperature gradient using these methods becomes more and more difficult with large single crystals.
In view of knowledge acquired to date, there is a demand, in particular with regard to crystal diameters of 200 mm and above, to find new economic methods for setting the required growth conditions, so that the defect profile required by the customer is obtained. Semiconductor wafers which include only COPs, in particular those with a predetermined size and density distribution, and semiconductor wafers which do not have any agglomerates of point defects, are of particular interest in this context. However, semiconductor wafers with a stacking fault ring (ring wafers), having both or having just one type of point defect may also be specified by the customer. The requirement is in particular for the growth conditions to be set in such a way that as many semiconductor wafers as possible having the specified defect properties can be separated from the single crystal.
The targeted control of the radial profile of the axial temperature gradient G(r) at the solidification interface and of the growth rate v not only makes it possible to set specific defect distributions in the single crystal. In addition, since the incorporation of oxygen and dopants in the single crystal is likewise highly dependent on the growth limit, targeted control of the temperature gradient also makes it possible to reduce radial variations of dopant and oxygen distributions.
One possible way of controlling this is to use magnetic fields when pulling the single crystal, since magnetic fields can be used to influence the flow conditions in the melt and therefore the temperature balance, in particular in the region of the solidification interface. Descriptions have been given of the use of static magnetic fields (horizontal, vertical and CUSP magnetic fields), single-phase or multiphase alternating fields, rotating magnetic fields and traveling magnetic fields. For example, according to patent applications EP 1225255 A1 and U.S. Pat. No. 2002/0092461 A1, a traveling magnetic field is used to enable the incorporation of oxygen in the single crystal to be controlled.